1. Field of the Invention
The present invention generally relates to a via structure in multi-layer substrate, and more particularly to a via structure in multi-layer substrate and/or in a flexible multi-layer substrate.
2. Description of Prior Art
Miniaturization of all electronic products is an inevitable trend in this modern world. While the scales of the semiconductor chips continuously get smaller, the scale of the related technology for packaging needs to be microminiaturized to follow the scale of the semiconductor chip is also inevitably getting smaller. Today, because the routing density of integrated circuits has been greatly increased, using a multi-layer substrate as being a package substrate, a print circuit board, a flexible package substrate or a flexible print circuit board and for obtaining a high performance integration system must be developed consequentially. Particularly, the flexible multi-layer substrate should be mentioned to become efficiently adoptable in variable electronic products for miniaturization. Generally, the metal lines and the via structures of the multi-layer substrate are manufactured by etching or semi-additive methods according to prior arts. The higher the circuit routing density increases, the finer the metal line and the via structure are requested. The fine-pitch products well known in the related industry field are generally meant to be high integration products.
Please refer to FIG. 1, which depicts a diagram of a via structure in a multi-layer substrate according to prior arts. FIG. 1 only shows related parts of the multi-layer substrate with the via structure. The multi-layer substrate comprises a metal layer 102 as being the lower metal lines. A dielectric layer 104 covers metal layer 102. The via 106 is formed by laser or mechanical drill. The via 106 has a vertical via wall. After the via 106 is formed and metal material is filled therein. With a metal layer 108 as a via land, the electrical connection between metal layer 102 and other upper metal layers of the dielectric layer 104 is objected to be established. When the multi-layer substrate comprises such kind of via structure and is applied to be bended. The metal layer 108 at the position 110 which is connected with the metal material in the via 106 and aligned to the edge of the dielectric layer 104 can be easily peeled off or even broken. Same issue can frequently happen at the position 112 where the metal layer 102 and the metal material in the via 106 connect.
The via structure is an important part for the electrical connection between different metal layers in the multi-layer substrate. When the via structure is manufactured in the flexible multi-layer substrate, and more particularly in the frequently bended area of the flexible multi-layer substrate, there is higher possibility that the position 110 of the metal layer 108 aligned to the edge of the dielectric layer 104 peels off or even breaks. The position 112 connecting the metal layer 102 and the metal material in the via 106 also peels off or even breaks with higher possibility, similarly. Meanwhile, the via land size A of such via structure has to be larger than the via diameter B. In consequence of limitation about the via pitches and metal line pitches of the multi-layer substrate, application to the high integration products will be failed.
Please refer to FIG. 2, which depicts a diagram of another via structure in a multi-layer substrate according to prior arts. Similarly, a metal layer 202 is formed and prepared for manufacturing the lower metal lines. A dielectric layer 204 covers metal layer 202 and includes the via 206. After the via 206 is formed and then via land is formed therein to establish the electrical connection between metal layer 202 and other metal layers above the dielectric layer 204. When the multi-layer substrate is frequently bended, the position 212 connecting the metal layer 202 and via land peels off with higher possibility. Especially when the via depth C becomes deeper, the possibility of the position 212 peeling off may be almost the same as that of the position 112 peeling off shown in FIG. 1. Meanwhile, as the via structures of the multi-layer substrate are manufactured by etching or semi-additive methods according to prior arts, the process tolerance has to be further considered (More details will be described later). Therefore, the via land size A of such kind of via structure has to be larger than the via diameter B. Still, in consequence of limitation about the via pitches and metal line pitches of the multi-layer substrate, further application to the higher routing density today cannot be realized.
Please refer to FIG. 3A and FIG. 3B, which depict diagrams of manufacturing a via structure by etching method according to prior arts at the same time. A metal layer 302 is formed in advance when the via structure is manufactured by etching method. The dielectric layer 304 is formed and covers the metal layer 302. Then, a via 306 is formed. Next, a metal layer 308 for manufacturing the via land 300 is formed to fill in the via 306. Meanwhile, the metal layer 308 also covers the dielectric layer 304. Afterward, a photoresist layer 310 is coated at the position of manufacturing via land. After the photoresist layer 310 is coated, the uncovered metal layer 308 with the photoresist layer 310 is etched and removed.
Because the etching is isotropic, not only the uncovered metal layer 308 is etched but the side surface of the via land is also etched. For example, the via land will be etched to shrink back to the position of dot lines 314. Therefore, an undercut structure happens to the via land as shown in FIG. 3. If the etching method is employed to manufacture the via structures of the multi-layer substrate, the photoresist layer 310 cannot determine the via land size exactly. Accordingly, the via land size cannot be further minified because the design tolerance of the etching process has to be considered. As finer the metal lines and sizes of the via structures are required, the etching method has limitation and cannot satisfy coming demands of the multi-layer substrate products.
Please refer to FIG. 4A to FIG. 4C, which depict diagrams of manufacturing a via structure by SAP, semi-additive process. First, a metal layer 402 is formed and then the dielectric layer 404 is formed to cover the metal layer 402 when the via structure is manufactured by etching method. Then, a via 406 is formed. Next, a seed metal layer 407 is formed. Afterward, a photoresist layer 410 is coated except the position of manufacturing via land. And then, a metal layer is formed to fill in the position of the via land. The seed metal layer 407 except the position of the via land 400 is removed by the etching method after the photoresist layer 410 is removed as shown in FIG. 4B and FIG. 4C to finish the process of manufacturing the via structure.
However, the via land 400 will be etched and shrinks back to the position of dot lines 414 as shown in FIG. 4B while the seed metal layer 407 except the position of the via land 400 is removed. Consequently, the via land 400 which's size is smaller than what the photoresist layer 410 originally defines is formed as shown in FIG. 4C. Similar drawback appears, if the semi-additive process method is employed to manufacture the via structures of the multi-layer substrate, the photoresist layer 410 cannot determine the via land 400 size exactly. Because the design tolerance of the semi-additive process has to be further considered and via land size accordingly cannot be minified. As finer the metal lines and sizes of the via structures are required strictly, the semi-additive method also has limitation and cannot satisfy coming demands of the multi-layer substrate products today and in the near future.
In conclusion, there is a need to develop a via structure in a multi-layer substrate and manufacturing method which the via land is inside the via to diminish the possibility of aforesaid peeling off or even breaking. The manufacturing tolerances of metal lines and via lands of the multi-layer substrate can be further reduced. Therefore, routing density can be increased and an electrical system can be easily integrated in more functions on a substrate. Moreover, the via structure in a multi-layer substrate can be applied to the flexible multi-layer substrates and raise the reliability of the package substrates.